X-ray crystallographic techniques are being used to study the structures of some carcinogenic polycyclic aromatic hydrocarbons and their activated metabolites, the structures of some polycylic mutagens, and covalent products of the metabolites with nucleic acid bases, low molecular weight polynucleotides and peptides. The aim is to obtain information relevant to the interaction of an activated carcinogenic polycyclic aromatic hydrocarbon with its target that is critical to carcinogenesis. The mechanisms of action of carcinogen-metabolizing enzymes will be investigated by structural studies of substrates, inhibitors, and models of the active sites. We will also attempt to extract and purify some of these enzymes. We will determine the high resolution structure of the enzmye, zylose isomerase, together with the amino acid sequence of this protein. a low resolution electron density map has already been computed. In addition, attempts will be made to purify and crystallize the enzyme aconitase from mammalian and bacterial sources with the aim of obtaining the full three-dimensional structure. Mechanisms of enzyme-catalyzed reactions will be investigated by X-ray structural studies of the analogues of coenzymes B12 and biotin. Also, structures of substrates and inhibitors of enzymes, such as citrate enzymes, will be determined.